John, so much depends on the actual area the pen is located. As Step said, more insulation will not hurt. But is the area safe from ice build up? If the ground freezes solid due to standing water from a roof drip line, or water pooling up-it could be bad for such a small turtle. Have you hibernated turtles before in this area? If water is an issue-you may want to remove the turtle and hibernate it in a milder environment that you can control.
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Tess Cook
www.boxturtlesite.info